CHDA is useful as a starting material for medicines, synthetic resins, synthetic fibers, dyes, and the like. In particular, t-CHDA is useful as a starting material in producing resins and fibers excellent in heat resistance, weatherability, physical strength, etc. There is a desire for CHDA having a high t-CHDA concentration.
A method generally employed for producing CHDA is to hydrogenate the benzene ring of a TPA derivative. For example, use is being made of a method which comprises converting the carboxyl groups of TPA into a metal salt, e.g., sodium salt, or into any of various esters before hydrogenating the benzene ring (nucleus hydrogenation) and a method in which TPA having the carboxyl groups is subjected to nucleus hydrogenation.
(1) Processes for CHDA Production
Known processes for producing CHDA through the step of hydrogenating not a TPA derivative but TPA include the following.    (i) A process comprising hydrogenating TPA in a solvent for TPA at 150 to 300° C. and at least 1,000 p.s.i.g. using a palladium catalyst to obtain crude CHDA, dissolving the crude CHDA in an aqueous alkali solution, and then conducting precipitation with an acid to purify (see patent document 1).    (ii) A process comprising hydrogenating TPA under the conditions of 150° C. and 100 kG in the presence of palladium or ruthenium, filtering the resultant liquid reaction mixture under specific temperature conditions, and crystallizing a CHDA from the filtrate (see patent document 2).    (iii) A process comprising hydrogenating TPA in a glass autoclave at 130° C. and a hydrogen pressure of from 8.3 to 9.8 kg/cm2 in a water solvent in the presence of palladium and subjecting the liquid reaction mixture to steam distillation to purify (see patent document 3).
The CHDA obtained by those production processes have had a low purity. In the process (i) in which purification is conducted by precipitation with an acid, inclusion of inorganic salts derived from sodium hydroxide, hydrochloric acid, and the like is unavoidable. In the process (ii) involving crystallization, inclusion of TPA, which is used as the starting material, and of by-products including trans-4-methylcyclohexanecarboxylic acid (hereinafter referred to as “t-MCHA”), cis-4-methylcyclohexanecarboxylic acid (hereinafter referred to as “c-MCHA”), and cyclohexanecarboxylic acid (hereinafter referred to as “CHA”) occurs. Because of these, it has been impossible to obtain a high-purity CHDA by either of these processes (i) and (ii). The process (iii) involving steam distillation requires a large amount of steam and necessitates wastewater treatment facilities. The process (iii) hence has had a problem concerning profitability.
Furthermore, in those processes, the hydrogenation of the benzene ring of TPA yields isomers and, hence, the CHDA obtained is a mixture of c-CHDA (melting point, 170-171° C.) and t-CHDA (melting point, 312-313° C.). The concentration of t-CHDA, which is the target compound, is as low as about from 20 to 50%, although it depends on reaction conditions. Consequently, techniques for improving the concentration of t-CHDA after CHDA are obtained from TPA are being investigated. A known technique for heightening the concentration of t-CHDA is to heat c-CHDA and thereby isomerize it into t-CHDA.
(2) Thermal Isomerization of CHDA
    (i) A method is known in which c-CHDA is heated to 250° C. or higher, preferably to the melting temperature of t-CHDA (310-313° C.) or a higher temperature, to obtain t-CHDA (see patent document 4). In an Example of patent document 4, there is a statement to the effect that t-CHDA of 98% was obtained by a method comprising heating a mixture of c-CHDA and t-CHDA to 310-320° C., holding it for 5 minutes, subsequently cooling the resultant homogeneous melt to ordinary temperature, and recrystallizing the reaction product from water using activated carbon.
This method described in patent document 4 is unclear with respect to the concentration of t-CHDA after the heating and before the recrystallization. The heating of the reactant is presumed to be conducted in air because the Example contains no particular statement concerning an atmosphere for reactant heating. It is hence thought that the CHDA is oxidized to generate impurities. Furthermore, since the mixture is heated to a temperature not lower than the melting point of the t-isomer, the t-CHDA obtained is exceedingly hard and is difficult to handle. In addition, t-CHDA having a high purity of 98% can be finally obtained only when t-CHDA is recrystallized from water using activated carbon after the heat treatment.
Namely, the method described in patent document 4 necessitates a two-stage operation, which is complicated. The present inventors followed up the method described in patent document 4. As a result, it was found that when the mixture which has been heat-treated at a temperature not lower than the melting point of t-CHDA is cooled to room temperature, the CHDA obtained is exceedingly hard and are difficult to handle and the concentration of the t-CHDA obtained through an isomerization reaction is low. Furthermore, the reactor was highly corrodible.
Known as a technique for improving handleability is (ii) a method which comprises heating and isomerizing c-CHDA at a temperature of 250° C. or higher to obtain t-CHDA, mixing the t-CHDA with an inert liquid substance to prepare a suspension, and obtaining the t-CHDA therefrom (see patent document 5). In an Example of patent document 5, a procedure is described which comprises keeping c-CHDA molten at 300° C. in a nitrogen atmosphere for 30 minutes, subsequently adding a liquid paraffin thereto, cooling the mixture to room temperature, and separating the resultant slurry, followed by washing with butanol and water to obtain t-CHDA having a purity of 99.5%.
In the method described in patent document 5, it is necessary to elevate the temperature of the reaction system to the melting temperature or higher and to disperse the melt with a liquid paraffin. It is also necessary to conduct washing with butanol and water in order to remove the liquid paraffin from the t-CHDA dispersed. In addition, it is thought that to completely remove the liquid paraffin by the washing is difficult.
(3) Thermal Isomerization of Aqueous CHDA Solution
A method is known which comprises heating an aqueous solution of c-CHDA to 240° C. or higher under pressure to thereby obtain t-CHDA (see patent document 6). In an Example of patent document 6, there is a statement to the effect that t-CHDA was obtained in a yield of 58.9% by heating an aqueous solution of c-CHDA at 245 to 250° C. in a nitrogen atmosphere for 2 hours, cooling the solution, subsequently filtering the resultant slurry at 70° C., and cleaning the solid with hot water.
In the method described in patent document 6, c-CHDA is isomerized into t-CHDA in an aqueous solution. However, this reaction in an aqueous solution is effective only in heightening the proportion of t-CHDA to about 60%, and about 40% of the c-CHDA remains without being isomerized.
(4) Thermal Isomerization with Alkali Metal Salt
A method is known in which an alkali (alkaline earth) metal salt of a c/t-CHDA mixture is heated in a solid phase in the presence of an alkali metal hydroxide or alkaline earth metal hydroxide to obtain t-CHDA (see patent document 7). In an Example of patent document 7, a procedure is described which comprises concentrating a mixture of c-CHDA, sodium hydroxide, and water to dryness under reduced pressure, heating the resultant solid at 200° C. in a sealed tube for 1 hour, cooling the resultant mixture, subsequently dissolving it in water, and then subjecting the solution to acid precipitation with hydrochloric acid to obtain t-CHDA in a yield of 95%.
In the method described in patent document 7, the alkali (alkaline earth) metal salt is subjected to an isomerization reaction in the presence of an alkali metal hydroxide or alkaline earth metal hydroxide. It is therefore necessary that after completion of the reaction, the reaction product should be dissolved and converted to the carboxylic acid through precipitation with an acid. Furthermore, it is presumed that in this method, the alkali (alkaline earth) metal comes as an impurity into the reaction product.    [Patent Document 1]
U.S. Pat. No. 2,888,484    [Patent Document 2]
JP-A-58-198439    [Patent Document 3]
JP-A-6-184041    [Patent Document 4]
JP-B-39-27244    [Patent Document 5]
JP-A-49-81349    [Patent Document 6]
JP-A-49-82648    [Patent Document 7]
JP-A-58-24540
As described above, the related-art techniques in which c-CHDA is isomerized into t-CHDA necessitate a complicated operation. Furthermore, in the case of obtaining the target compound by mere heating by a simple method, difficulties have been encounted in handling the CHDA's resulting from the isomerization reaction. There has hence been a desire for a method for obtaining t-CHDA in a high concentration from c-CHDA with high productivity.
In addition, in the related-art techniques for obtaining t-CHDA, a TPA derivative which can be easily hydrogenated industrially has been used as a starting-material TPA for obtaining crude CHDA therefrom. Namely, when a metal salt of TPA is used as a starting-material TPA, the metal, e.g., sodium, remains. When a TPA ester is used, the ester is first converted into a salt and then subjected to precipitation with an acid. Because of these, polymers obtained from this t-CHDA have had the following problems: (1) they contain metallic impurities derived from the metal salt of TPA; (2) a component of the acid used for the acid precipitation, e.g., chlorine or sulfur, remains; (3) the residual acid component corrodes apparatus and others; and (4) the polymers have a low value of transmittance at 340 nm, which property is an index to the degree of coloring. High-quality t-CHDA has been desired.